The game, although looks continuous, is actually discrete. There are 25 distinct frame per second, and every action takes a certain number of frames. When you reach a speed breakpoint, the action is done 1 frame less.
For example, a sorc has breakpoints at 63 and 105. So, if you go from 62 to 63, you get 1 frame faster. If you go from 104 to 105, again, 1 frame faster. If you go from 70 to 71, or even from 63 to 104, you don't get any speed ups.